The present invention relates to a method for forming electrophotographically duplicated images with the aid of a photosensitive screen having a great number of fine apertures.
An electrophotographic process using a photosensitive screen has been known from, for example U.S. Pat. No. 3,680,954. In this known process use is made of a photosensitive screen comprising an electrically conductive mesh-like member having a photoconductive layer applied thereon. This photosensitive screen has several drawbacks in that a primary electrostatic latent image formed on the photoconductive layer cannot have a high surface potential and the electrostatic latent image cannot be maintained stably for a long time period due to a relatively large dark decay. Therefore it is practically difficult to form a number of secondary electrostatic latent images with the aid of the single primary latent image once formed on the screen.
In Japanese Laid-open Patent Application No. 341/76 there is described another electrophotographic process. In this method the primary latent image is formed on the photosensitive screen by means of the following successive steps; i.e. a primary voltage application step for uniformly charging the photosensitive screen, a step for projecting an optical image of a document to be duplicated on to the screen, and a secondary voltage application step for changing the surface potential of the screen in accordance with the projected pattern. After the primary latent image has been formed in the manner mentioned above, an ion stream is passed through the screen from an ion source and is modulated in accordance with the primary latent image on the screen so as to form a secondary electrostatic latent image on a record medium such as a record paper.
FIG. 1 is a cross sectional view showing the photosensitive screen described in the above mentioned Japanese Laid-open Patent Application. The screen 1 comprises a conductive substrate 2 having a great number of fine apertures, a photoconductive layer 3 applied on a part of the substrate 2 and an insulating layer 4 applied on the photoconductive layer 3. In the primary voltage application step a corona wire 5 is arranged opposite to the insulating layer 4 and a corona voltage supply source 6 is connected across the corona wire 5 and the conductive substrate 2 of the screen 1 as shown in FIG. 2. On the other hand if the corona discharge is effected from the opposite side, the corona ion stream might flow into the substrate 2 and thus the insulating layer 4 could not be sufficiently charged.
Next as illustrated in FIG. 3 with respect to the screen 1 having the primary charge, an imagewise exposure step and a secondary voltage application step are effected simultaneously. That is to say a corona wire 7 is arranged opposite to the insulating layer 4 and a document 8 to be copied is arranged behind the corona wire 7. While a light image of the document 8 is projected on to the screen a D.C. voltage 9 having an A.C. voltage 10 superposed thereon is applied across the conductive member 2 of the screen 1 and the corona wire 7. Then primary latent image is formed on the insulating layer 4 of screen 1.
Then as shown in FIG. 4 the screen 1 is illuminated with uniform light. By this uniform exposure electrostatic charge is formed on the insulating layer 4 in such a manner that at the dark portion of the light image an electric field is formed for enhancing or accelerating the passage of the ion stream through the apertures of the screen, and at the bright portion an electric field for preventing or blocking the passage of ion stream through the screen are formed, respectively. In this manner a primary latent image having a high contrast is formed on the photosensitive screen 1.
FIG. 5 illustrates a step for forming a secondary latent image on a record member by modulating the corona ion stream in accordance with the primary latent image formed on the screen. To this end a corona wire 11 is arranged opposite to the conductive substate 2 and a counter or back electrode 12 is provided on which electrode is placed a record member 13. Across the corona wire 11 and the back electrode 12 are connected a corona voltage source 14 and an accelerating voltage source 15 in such a manner that a potential difference is produced from the corona wire 11 toward the screen 1 and back electrode 12. Then the corona ion stream is forced to flow from the corona wire 11 toward the record member 13. However at the bright portion of the primary charge image on the screen 1 there is formed the field .alpha. having the direction for blocking the passage of corona ion stream (shown by broken lines) and thus the corona ion does not pass through the screen, but flows into the exposed conductive substrate 2. Whilst at the dark portion of the primary image on screen 1, since there is formed the field .beta. having the direction for accelerating the corona ion stream the ion stream is forced to pass through the screen and reaches the record member 13. In this manner on the record member 13 there is formed a secondary latent image corresponding to the primary latent image formed on the screen 1.
In the above mentioned known electrophotographic process the electrostatic charges on the screen forming the primary latent image are balanced with respect to the electrostatic charge of opposite polarity on screen 1.
In the above mentioned known electrophotographic process, since there are formed electrostatic charges of opposite polarities on respective surfaces of the insulating layer 4 and these charges are balanced electrostatically, the charge on the insulating layer 4 is scarcely cancelled by the corona ion for forming the secondary latent image even if the corona ion has the opposite polarity to that of the primary latent image formed on the insulating layer 4. Therefore it is possible to form a number of the secondary latent images by repeatedly subjecting the secondary latent image forming step to the primary latent image once formed on the screen 1.
However as shown in FIG. 2 even if the corona charging is effected from the insulating layer side, a relatively large part of the corona ions practically flows into the exposed conductive substrate 2 and thus the potential on the insulating layer 4 would not be sufficiently high. Therefore the contrast of the primary latent image obtained after the uniform exposure step shown in FIG. 4 is not high, and thus only a limited number of duplicated copies of good quality can be obtained from the single primary latent image once formed on the screen 1.
In U.S. patent application Ser. No. 19,787, filed on Nov. 11, 1971 there is described still another known electrophotographic process. A photosensitive screen for use in this known process is of a four-layer construction and comprises a conductive mesh-like sheet member, a photoconductive layer applied on one side of the mesh member, an insulating layer applied on the other side of the conductive member and a conductive layer applied on the insulating layer. Since the primary latent image is formed on the photoconductive layer the dark decay is large and a primary latent image of good quality can not be maintained for a long time period as in the case of the first mentioned known electrophotographic process. Moreover, there might occur a spark due to the breakdown in the insulating layer upon applying a bias voltage across the conductive substrate and conductive layer. Further, the photosensitive screen of four-layer construction is very complicated, and it is quite difficult to manufacture such a screen.